Question: We are measuring CO2 as an indicator of proper ventilation to reduce COVID transmission risk. Has anyone considered measuring relative humidity (from exhaled aerosols and droplets) as another indicator of risk?

crispinpierce is asking a question about general
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by crispinpierce | August 10, 2021 19:08 | #27453

We're interested in measuring classroom ventilation to control COVID transmission risk. While this virus (like other respiratory viruses) appears to be less viable in more humid atmospheres, these conditions are also caused by breath exhalation (gym windows and mirrors fogging up). Could measurement of increases in relative humidity indoors reflect inadequate ventilation?


Hi Dr. Pierce! Thanks for posing the question 👍 Indoor ventilation, while not the only aspect in transmission, is definitely an important one. CO2 and humidity are useful metrics in that they can indicate the adequacy of the ventilation to the number of people in the room. One challenge of using humidity would be the attribution to the occupants (i.e. how could one be confident that the humidity is due to breathing and not a leaky faucet?), and some challenges for CO2 monitoring would be the questionable reliability of low-cost CO2 sensors, the nonlinear relationship of transmission rates and CO2 concentration (e.g. CO2 concentration would not be impacted by mask wearing but transmission would).

However, in this presentation, Dr. Peng Xu recommends using CO2 as an indicator of indoor virus concentration, though there are several other methods listed in this paper to measure ventilation rate. I found this paper to be helpful in identifying common pitfalls and recommendations for measuring ventilation.

Dr. Xu also provides guidance for ACH rates by building type, estimates the probability of infection for various HVAC system types around the 18-min mark, and presents low cost strategies for reducing transmission risk later in the presentation.

Hopefully some of these thoughts and links are helpful!

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